Universal quantum-copying machines: A sufficient and necessary condition

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Journal Article
Physics Letters, Section A: General, Atomic and Solid State Physics, 2002, 302 (1), pp. 1 - 7
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It was shown that the copying quality of the Wootters-Zurek quantum copying machine depends on the input states. So Buzek and Hillery proposed the notion of universal copying machine. A universal machine is a quantum copier whose copying quality is independent of the input states. Buzek and Hillery further introduced a class of universal copying machines. In this short Letter, we establish a sufficient and necessary condition for a general copying machine to be universal. This condition will degenerate to the Buzek and Hillery's original one when we use it to analyze the Buzek and Hillery machines. On the other hand, this result enables us to find some new universal quantum-copying machines, and it also helps us to derive some interesting properties of universal machines. © 2002 Published by Elsevier Science B.V.
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